This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art. The abundance of methane, the main component of natural gas (˜95%) and shale gas (typically >70%), on Earth makes it an attractive source for energy and chemicals for at least the next century. Catalytic transformation of methane to value-added chemicals plays an important role in methane utilization. Various routes have been considered, including indirect transformation which converts methane to syngas as intermediate followed by its further conversion to other compounds, and direct transformation which converts methane to higher hydrocarbons (e.g. ethylene, benzene) or oxygenates (e.g. methanol, formaldehyde) without any intermediate products. The direct transformation is more attractive because it saves both operating and capital costs. Among direct transformation technologies, oxidative coupling of methane (OCM) is promising because the primary products (C2 species, ethane/ethylene) are precursors for a variety more high valuable products, e.g. plastics and resins. (The term coupling is well understood by chemists, chemical engineers and those skilled in the art. A coupling reaction in organic chemistry is a general term for a variety of reactions where two hydrocarbon fragments are coupled with the aid of a metal catalyst.) Tuning the selectivity towards C2 species in OCM, however, has been a long-standing challenge since the 1980s, owing to the unavoidable presence of over-oxidized species (CO/CO2) under oxidative conditions. (The term selectivity is well understood by those skilled in the art and generally refers to a measure of desired product formed in relation or ratio to all products—desired and undesired—formed. Other terms well understood include “conversion” which is a measure of the amount of the reactant that reacted, and “yield” which is a measure of the desired product formed in relation to the reactants. Hundreds of catalyst candidates have been prepared and tested for OCM, while carbon selectivity towards CO/CO2 is typically about 50%, indicating uneconomic conversion of carbon atoms. Non-oxidative conversion of methane, first reported in 1993, improves carbon atom economy. Using Mo supported on zeolites, existing non-oxidative technologies generate benzene as the main product, but unavoidable coke formation limits catalyst lifetime and process commercialization. Although the selectivity toward benzene is typically about 80-90%, other aromatic hydrocarbons (C7-C9) as well as C2 species (both ethane and ethylene) have also been reported. In a recent report, 2-3% methane conversion was reached over Bi/SiO2 at 900° C. under non-oxidative conditions, while the selectivity toward C2 products was about 40%.
Non-oxidative coupling of methane (NOCM) to form C2 hydrocarbons has been considered since the 1990s. It has been reported that C2H6 and H2 were immediately produced when CH4 was fed continuously over a commercial 6% wt Pt/SiO2 catalyst at low temperature 250° C., while owing to catalyst deactivation, both products disappeared for time on stream (TOS) more than 8 min. This indicates that methane can be activated at temperature lower than typically used in OCM (>700° C.). Other researchers showed that ethane with >98% carbon selectivity was produced over silica-supported tantalum hydride catalyst at temperature <500° C., although methane conversion was less than 0.5%. Others reported 48% conversion of methane under non-oxidative condition over Fe/SiO2 catalyst at 950° C., producing ethylene, benzene and naphthalene with carbon selectivity of 53%, 22% and 25%, respectively. Some other researchers found similar products over Pt—Sn catalyst at 700° C.; however, the methane conversion was less than 0.3%.
As an important direct methane transformation technique, despite extensive research conducted for decades, oxidative coupling of methane (OCM) remains industrially uneconomic owing to low carbon selectivity (typically <50%) towards valuable target products (C2 species, ethane/ethylene). Thus, there is an unmet need for methods and processes to produce C2 hydrocarbons from methane in non-oxidative conditions with both high carbon selectivity and high conversion.